Macrocyclic oligoesters, also called macrocyclic polyester oligomer compositions, can be converted, often under isothermal conditions, to linear polyesters of high crystallinity and solvent resistance.
One method for preparing macrocyclic oligoesters is accomplished by the reaction of diols, such as ethylene glycol or 1,4-butanediol, with dicarboxylic acid chlorides, such as terephthaloyl chloride or isophthaloyl chloride, under specifically defined conditions. Other methods of preparing macrocyclic polyester oligomer compositions include the reaction of a dicarboxylic acid chloride, such as terephthaloyl chloride, with a bis(hydroxyalkyl) dicarboxylate, such as bis(4-hydroxybutyl) terephthalate.
Another macrocyclic oligoester preparation method is catalytic depolymerization of linear polyesters such as poly(1,4-butylene terephthalate) (“PBT”) and poly(ethylene terephthalate) (“PET”). Catalytic depolymerization macrocyclic oligoester preparation methods require that linear polyesters be purchased or manufactured prior to macrocyclic oligoester production. Producing macrocyclic oligoesters from high molecular weight linear polyesters necessitates handling of a high molecular weight material. The high molecular weight linear polyester material typically has a high viscosity, which requires costly equipment. In some instances it also requires many expensive finishing steps.
For example, prior art methods employ melt reactors that are capable of generating surface area in high viscosity high molecular weight material. Where poly(butylene terephtalate) is reacted with 1,4-butanediol, it is necessary to generate surface area to enable the diol to diffuse to the surface so that the reaction may proceed to build the molecular weight of the polymer. When high viscosity high molecular weight materials are prepared, diffusion of the diol from the polymer matrix is rate limiting and generating surface area by employing a melt reactor increases the reaction rate of the process of building the molecular weight of the polymer. Such melt reactors are highly engineered, energy intensive, and are run under relatively high vacuum conditions (e.g., 0.5 Torr) that are necessary to handle the high viscosity material.
Further, U.S. Pat. No. 4,590,259 to Kosky et al. describes a method for preparing poly(alkylene terephthalates) of blow molding grade in which a final step is conducted under conditions necessitated by the high melt viscosity of high molecular weight polyesters. A prepolymer preparation step is described in which, for example, a poly(butylene terephtalate) undergoes reaction with 1,4-butanediol. The resulting prepolymer undergoes reaction further with the diol in the presence of an inert gas to reduce the number of acid end groups, for example carboxylic acid end groups, to a desired level. Thereafter, solid state polymerization is conducted whereby the molecular weight of the polymer is increased under high vacuum conditions.